There are many different types of submersible pumps including for example, sump, effluent, and sewage pumps, that have internal electric motors housed in or near the pumpage. Such pumps must often incorporate several key features that enable them to function safely in harsh environments. These key features include but are not limited to: an isolated motor chamber to house the electric pump motor and other electrical devices; a liquid tight cord entry for electrical power access into an isolated motor chamber; an electrical grounding feature; and accessories that can include liquid level controls, seal sensors, and alarms.
Submersible pumps must be designed with features that provide adequate protection to electrical devices housed in or near the pumpage. One such feature is the motor housing, which typically is made of a metal, preferably cast iron, that affords protection from impact, abrasion, and pumpage infiltration.
When the motor housing is attached to mating components like a seal housing or a motor adapter, an isolated area within the submersible pump assembly is formed. This isolated area is called “the motor chamber” and is an area where the electric pump motor and related electrical components are located isolated from the pumpage.
One method for providing further protection to the electrical components is to have the motor chamber filled, in whole or in part, with an electrically inert liquid, often referred to as “turbine oil”. Turbine oil is mainly used for heat dissipation, bearing lubrication, and emulsification of pumpage infiltration. Emulsification is known by those skilled in the pump art to extend a pump's operating life by delaying potential short circuiting should a seal failure occur and inadvertently allow electrically conductive liquids into the motor chamber.
In order to effectively operate a submersible pump in or near the pumpage, liquid tight access for the passage of electric power and accessory cables entering into the isolated motor chamber of the pump must be provided. A typical cord entry has two functions: 1) securing the cable or cordage from excessive movement that could cause damage; and 2) preventing the flow of liquids in and around electrical components and motor bearings. There are two types of cord entries commonly used on submersible pumps that provide these functions. They are a “cord grip” and a “detachable cordset”.
FIG. 1 shows a cord grip 10, which is a type of cord entry that seals or isolates the external pumpage from entering the motor chamber of a pump by incorporating a grouping of pliable packing material 11, a metal washer 12, a gland housing 13, and a gland nut 14. The gland housing 13 is typically integrated into the motor housing (not shown) or attached by mechanical means using one or more of the following methods: a tapered pipefitting 15, fasteners 16, and an O-ring 17, gasket and/or epoxy. When properly assembled, the gland housing 13 controls the forces exerted by the gland nut 14 onto the pliable packing material 11, and in particular, to exert enough force to cause the packing material 11 to flow and crush against a power-cord jacket 18, thereby creating a water tight seal. Because of the forces imparted onto the power-cord jacket 18, a certain amount of cord anti-slippage or pullout resistance is offered, hence the name “cord grip”.
FIG. 2 shows a detachable cordset 20, which is a type of cord entry having a power cord that is detachable from the pump to improve serviceability. A typical detachable cordset consists of a receptacle, an over-molded plug, a washer, and a compression nut. The receptacle portion of the detachable cordset is typically configured to have one half of a mating set of pins that interface with the over-molded plug, which has either a corresponding engaging or receiving set of mating pin connections. The over-molded plug portion of the detachable cordset is typically molded from a compressible electrically insulating material that works similar to the packing gland of the cord grip by compressing and sealing around the conductors when pressure is applied by a gland nut of the detachable cordset. The receptacle housing of the detachable cordset is either integrated into the pump's motor housing or mechanically attached by some sealable means e.g., pipe fittings, fasteners, o-rings, gaskets, and/or epoxies.
Referring to FIG. 3, safety agency protocol requires that whenever electrically conductive material is used to enclose potentially live electric components, an electrical grounding feature including for example, a ground lead 30, ground ring terminal 31, and ground screw 32 arrangement, must be incorporated to avoid the possibility of electric shock. Because submersible pumps typically have metal motor housings to enclose the motor windings, such an electrical grounding feature is used. In most cases, the motor housing 33 will be drilled and tapped in order to receive the ground screw 32, which will be used to affix the ground ring terminal 31 attached to the ground lead 30 to the motor housing 33.
Typical submersible pump accessories may include liquid level control devices, alarms, and seal sensors. These accessories are typically critical to product performance and various pump functions like liquid transfer, liquid level monitoring, and product maintenance monitoring.
A liquid level control device is typically be used for the liquid level control of the pumpage. This device is usually mounted external to the pump but may have a power cord that passes through a cord entry (like the earlier described cord grip) and into the motor housing where the appropriate electrical connections are made within the motor chamber and isolated from the pumpage. Another common method of energizing this device is to use a separate power cable that runs tandem to the pump power cord and is terminated with a special type of plug end called a NEMA-5P piggyback plug. Once plugged into a power outlet, the piggyback plug will only allow power to be transmitted to the pump when activated by a switching device.
Similarly, a typical pump alarm may have electrical connections internal to the motor chamber or those that run tandem to the pumps' powercord. The alarm can send a signal to any number of devices to notify people of a particular function detected, weather it be a high liquid level setting off a bell or siren, or in some cases sending a message via a telephone to a service center or municipality.
Additionally, a seal sensor is commonly used in the pump art to detect a possible mechanical shaft seal or other type of failure by sensing a heightened level of moisture emulsified within the turbine oil. Persons of ordinary skill in the art will appreciate that if this condition is left unchecked, the moisture can have a detrimental effect upon the components located within the isolated motor chamber, i.e., the electric pump motor and motor bearings.
Submersible pumps with electrical devices mounted in or near the pumpage have shortcomings associated with: 1) maintaining an isolated motor chamber; 2) cord entries and “oil wicking”; 3) electrical grounding features; and 4) energizing accessories like liquid level control devices.
Isolating the electrical components of a typical submersible pump has always been a challenge for the pump industry. As previously mentioned, key features used to provide adequate protection to the electricals are the metal motor housing and the insulating turbine oil. Because of the need to bring powercords and accessory control cords into the isolated area of the motor chamber, access ports must be created in the motor housing. As with any gasketed, sealed and fitted joint, the access port created in the motor housing is at risk of being a potential leak path for gas or liquid infiltrating into or out of the isolated motor chamber.
In a typical submersible pump as shown in FIG. 4, multiple access ports 40 are commonly used to bring in the earlier described power cords, seal sensors and externally mounted liquid level control devices. These multiple access ports increase the risk of additional leak paths, which jeopardizes the integrity of the isolated motor chamber, and consequently, the pump. Multiple access ports also require costly machining operations, such as drilling, boring, milling, tapping, and costly joining techniques, such as bonding, in order to seal a typical cord entry.
With a typical submersible pump having cord entries exposed to the pumpage, it is extremely important that an integral seal be formed between the access port of the motor housing and the particular cord entry used to secure the power/accessory cord to the pump. The earlier described cord grip can offer adequate sealing between the motor housing and the outer jacket of a powercord passing through it, unfortunately, cord grips cannot block or prevent the flow of liquid internal to the powercord.
One of the undesirable consequence of using turbine oil in a submersible pump is the oil “wicking” effect. Because turbine oil has such a low viscosity, it tends to seep or wick-up into the individual strandings of the electrical conductors of the electric power cord due to capillary action. If left unchecked the capillary action could cause turbine oil to seep-out from the motor chamber.
As shown in FIG. 5, oil wicking is typically addressed in the prior art by potting the power cord. Power cord potting typically consists of a complex process that involves stripping away a small portion of each of the conductor's insulative jacketing and applying an epoxy 51 to the exposed conductors 50, which must be fully cured to prevent turbine oil wicking. It is common to have cure times up to 48 hours. This power cored potting process undesirably increases the cost of the power cord.
Another disadvantage associated with cord grips stems from the loss of sealing integrity that can occur from the relaxation of the compressed packing gland material. When a cord grip gland nut is torqued to a specified value, the compressed gland material, often butadiene rubber, relaxes and flows over time into the voids around the jacketed power cord. This causes both the power cord and gland nut to loosen, which may lead to failure if not corrected. Although, re-torqueing the gland nut after a designated time period will correct this problem, to do so requires additional handling which is undesirably costly, and delays packaging and shipping of the pump.
The earlier mentioned electrical ground feature adds further to the expense of the pump. Specifically, electrical grounding is typically accomplished via a grounding ring terminal connected to the ground lead of the incoming power cord that is mechanically fastened to the motor housing with a ground screw. The separate ground screw and lead, and the drilling and tapping that must be performed in the motor housing in order to implement the electrical ground feature, add to the expense of the pump. Another disadvantage associated with the electrical ground feature is that the ground wire can be pinched or damaged during the assembly process.
Economic disadvantages have been observed with variations in the configuration of the exterior mounted liquid level control that is commonly used on submersible pumps. The first disadvantage concerns the use of the separated powercord that follows tandem to the main pump powercord (often 10 ft. long) and terminates with a special plug called a NEMA-5P piggyback plug. The second disadvantage concerns the use of the short powercord emerging from within the pump's motor housing with termination located in the isolated motor chamber. This is perceived to be a disadvantage because of the difficulty and expense associated with its assembly.
An additional disadvantage associated with electrical connections originating within the motor chamber lies with the difficulty of servicing or replacing the electrical connections. Service or replacement of the electrical connections cannot be accomplished without draining the turbine oil and disassembling the pump.